of vitamin B12 (CN-B12) to coenzyme B12; five are of unknown function; and seven share similarity to genes involved carboxysome formation (Bobik et al. 1999).

      The carboxysome is a polyhedral organelle found in cyanobacteria and some chemoautotrophs consisting of a proteinaceous shell and interior. They contain the majority of the cell's RuBisCO, a major Calvin cycle enzyme, and are thought to actively participate in concentrating CO2. Similar bodies were observed in S. enterica cells grown on PD minimal medium and initial genetic analysis of the pdu operon indicated that genes encoded by the pdu operon are important for the formation of these polyhedral organelles (Bobik et al. 1999).

      Here we describe investigations on the conditions needed for the formation of the polyhedral organelles. In addition, a pool of TPOP insertion mutants is screened and genetic analyses conducted on a subset of mutants indicate that both the pduA and pduB genes are required for the formation of the polyhedral organelles. Formerly, diol dehydratase was shown to be associated with the polyhedral organelles of S. enterica ((Bobik et al. 1999)). Here, a nonpolar diol dehydratase mutant is constructed and is demonstrated to be capable of polyhedral organelle formation. In addition, complementation studies conducted on this strain demonstrate that diol dehydratase can be packaged into the polyhedral organelles of this strain from a plasmid encoded source; however it was not determined whether diol dehydratase was packaged previous to or after the formation of the organelle shell. The results are discussed in relation to the formation and function of the polyhedral organelles.

                              Materials and Methods

Chemicals and Reagents

      Ampicillin, fumarate, MgS04, pararosaniline, sodium cacodylate, sodium bisulfite,